A disk drive system is one type of magnetic data storage system. More specifically, a disk drive system is a digital data storage device that stores digital information within concentric tracks on a storage disk. The storage disk is coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a substantially constant rate. To read data from or write data to the disk, a magnetic transducer is centered above a desired track of the disk while the disk is spinning.
Writing is performed by delivering a write signal having a variable current to the transducer while the transducer is held close to the spinning track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track. The magnetic polarity transitions are representative of the data being stored.
Reading is performed by sensing the magnetic polarity transitions on the rotating track with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the magnetic signal into an analog read signal that is then delivered to a read channel for appropriate processing. The read channel typically includes an amplifier, an equalizer and a detector. A typical implementation of the read channel would divide the circuit between two chips: (1) the first including a preamplifier residing near the transducer, and (2) the second located on a printed circuit board (PCB) within the drive assembly. Within the read channel the analog read signal is converted into a properly timed digital signal that can be recognized by a host computer system external to the drive.
The transducer can include a single dual-purpose element, such as an inductive read/write element for use in both reading or writing, or it can include separate read and write elements. Transducers that include separate elements for reading and writing are known as "dual element heads" and usually include a read element which contains magnetoresistive (MR) material for performing the read function. Dual element heads are advantageous because both the read element and write element of the transducer can be optimized to perform their particular functions. For example, MR read elements are more sensitive to small variable magnetic fields than are inductive heads and, thus, can read much fainter magnetic polarity transitions from the disk surface. MR elements, however, are not capable of writing to the disk surface. Because MR elements are more sensitive, data can be more densely packed on the surface of the disk with no loss of read performance.
MR read elements generally include a strip of magnetoresistive material called a sensor that is held between two magnetic shields. The resistance of the magnetoresistive sensor varies almost linearly with an applied magnetic field. During a read operation, the MR strip is held near a desired track, specifically, within the varying magnetic field caused by the magnetic transitions on the track. A constant current is passed through the strip resulting in a variable voltage across the strip. By Ohm's law (i.e., V=I*R), the variable voltage is proportional to the varying resistance of the MR strip and, hence, is representative of the data stored within the desired track. The variable voltage signal (which is the analog read signal) is then processed and converted to digital form for use by the host.
Portions of a standard disk drive, generally designated 1, are illustrated in FIG. 1. The disk drive comprises a disk 4 that is rotated by a spin motor (not shown). The spin motor is mounted to a base plate (not shown). Data is stored on magnetic material 5 which coats the disk 4. An actuator arm assembly 7 is also mounted to the base plate.
The actuator arm assembly 7 includes a transducer 10 mounted to a microactuator arm 13 which is attached to an actuator arm 16. The actuator arm 16 rotates about a bearing assembly 19. The actuator arm assembly 7 cooperates with a voice-coil motor (VCM) 22 which moves the transducer 10 relative to the disk 4. The spin motor, voice-coil motor 22 and transducer 10 are coupled to a number of electronic circuits mounted to a printed circuit board (not shown). The electronic circuits typically include the read channel chips, a microprocessor-based controller and a random access memory (RAM) device, among other things.
Instead of having a single disk 4 as shown in FIG. 1, a disk drive 1 may include a plurality of disks as shown in FIG. 2. Each of the plurality of disks 4 has two sides as shown in FIG. 2 with magnetic material on each of those sides. Therefore, in the disk drive shown in FIG. 2, two actuator arm assemblies 7 are provided for each disk 4.
Referring again to FIG. 1, data is stored on the disk 4 within a number of concentric radial tracks 25 (or cylinders). Each track 25 is divided into a plurality of sectors, and each sector is further divided into a servo region 28 and a data region 31.
The servo regions 28 of the disk 4 are used to, among other things, accurately position transducer 10 so that data can be properly written onto and read from the disk 4. The data regions 31 are where non-servo related data (i.e., user data) is stored and retrieved. Such data, upon proper conditions, may be overwritten.
FIG. 3 shows a portion of a track 25 for a disk 4 drawn in a straight, rather than arcuate, fashion for ease of depiction. As is well-known, tracks 25 on magnetic disks 4 (as depicted in FIG. 1) are circular. It should be understood, however, that the track 25 shown in FIG. 3 could also represent tracks on magnetic tape, which tracks are generally straight. Each track 25 has a centerline 40. To accurately write data to and read data from the data region 31 of the disk 4, it is desirable to maintain the transducer 10 in a relatively fixed position with respect to a given track's centerline 40 during each of the writing and reading procedures.
With reference to FIGS. 1-3, to assist in controlling the position of the transducer (in this case an MR head) relative to the track centerline 40, the servo region 28 contains, among other things, servo information in the form of servo patterns comprised of one or more groups of servo bursts, as is well-known in the art. First and second servo bursts 46, 49 (commonly referred to as A and B servo bursts, respectively) are shown in FIG. 3. The servo bursts 46, 49 are accurately positioned relative to the centerline 40 of each track 25, and are typically written on the disk 4 during the manufacturing process using a servo track writer ("STW"). Unlike information in the data region 31, servo bursts 46, 49 may not be overwritten or erased during normal operation of the disk drive 1.
As the transducer 10 is positioned over a track 25, it reads the servo information contained in the servo regions 28 of the track 25, one servo region at a time. The servo information is used to, among other things, generate a position error signal (PES) as a function of the misalignment between the transducer 10 and the track centerline 40. The PES signals are input to a servo control loop which performs calculations and outputs a servo compensation signal which controls the voice-coil motor 22 to place the transducer 10 over the track centerline 40.
With reference to the system shown in FIGS. 1 and 3, it should be noted that the only time that the transducer 10 can be adjusted for track centering is when the transducer 10 reads the burst patterns 46, 49 in the servo region 28.
Tracks 25 are generally configured as concentric circles on the magnetic disk 4. As data is written onto the track 25, every effort is made to align the transducer 10 to the centerline 40 of the track. If this alignment were achieved, the centerline 40 of the track 25 would correspond to the portion of the track with the strongest magnetic signal (defined herein as the magnetic center of the track). Therefore in an ideal situation, the magnetic center of the track 25 would perfectly correspond to the physical centerline 40. Unfortunately, deviations from the centerline 40 occur during writing because of various tolerances and thermal expansion coefficients of the mechanical components which position the transducer 10 over the track 25. These inaccuracies in positioning the transducer 10 over the centerline 40 of the track 25 when data is written cause the magnetic center to not correspond with the dimensional centerline 40 of the track. During subsequent read operations where misalignment in writing has occurred, a transducer 10 which is positioned over the centerline 40 of the track 25 will not be positioned over the magnetic center. By failing to read from the magnetic center of the track (i.e., that portion of the track 25 which contains the strongest magnetic signal), BER may be increased (the BER being the number of errors versus the total number of bits read).
Deviations of the magnetic center from the physical centerline 40 become more prevalent when track widths are decreased, as is done in an effort to increase storage density of the magnetic media. So as track widths decrease, improved methods are needed to determine the magnetic center of the track 25 and move the transducer 10 to that magnetic center. Accordingly, there is a need for a disk drive system which is capable of reading from a location of a track based upon the strength of magnetic signal stored on the disk rather than solely a position relative to the track centerline.
Another problem with the conventional disk drive system 1 shown in FIGS. 1-2 is that, due to mechanical limitations, the actuator arm 16 moves the transducer 10 over the track centerline 40 very slowly (i.e., its response time is very slow). Accordingly, in conventional systems, the servo sector 28 must be of a sufficient length to allow the actuator arm 16 to move the transducer 10 over the track centerline 40 before reading the data from the data sector 31. In the alternative, the velocity of rotation of the disk 4 must be slowed to allow the transducer 10 to be centered on the track 25 before reading from the data sector 31. If sufficient time is not allowed for the transducer 10 to be centered over the track 25, the BER of the system will increase.
In order to increase response times, and, thus, reduce the BER, some have incorporated microactuator arms 13 on the ends of the actuator arms 16 as shown in FIGS. 1-2. The actuator arms 16 perform coarse positioning of the transducer 10, while the microactuator arms 13 perform fine position adjustments so that the transducer 10 is centered over the track. Because microactuator arms 13 have a smaller mass and are shorter in length, they may be moved more rapidly onto the track centerline 40 as compared to actuator arms 16. Accordingly, the transducer-centering response time may be decreased.
There are inherent mechanical problems, however, associated with microactuator arms 13. First, the microactuator arms 13 may be subject to wear due to vibrational affects. For example, as is well-known in the art, when a disk drive shuts down, the transducers 10 land in a landing zone. Vibrations may be felt throughout the microactuator arm, including at its point of connection to the actuator, during the landing process. Accordingly, the connection between the microactuator arm 13 and actuator arm 16 may wear over time due to the vibrations and impacts felt at each landing. Second, the assembly of a system containing microactuator arms 13 may be both costly and complex. Third, because microactuator arms 13 are positioned based upon a servo-compensation current and, therefore, are subject to sensitivity limitations, the incremental distance that the microactuator arm 13 needs to move to be properly positioned over the track may be unreachable due to its lack of sensitivity. Finally, and probably most importantly, because microactuator arms are mechanical systems, they are still subject to mechanical response times which are generally very slow as compared to those of electronic devices (albeit, microactuator arms' response times are much faster than those of actuator arms).
In view of the above, there is a need for a electronic device which is capable overcoming the mechanical disadvantages associated with microactuators.
Another problem associated with the disk drive system 1 of FIGS. 1-3 is that of thermal asperities. Thermal asperities are caused by foreign particles or aberrations on the surface of the disk.
As mentioned above, magnetic polarity transitions are stored on the disk 4 and, as the disk spins below the transducer 10, the magnetic polarity transitions on the disk present a varying magnetic field to the transducer. The transducer 10 is biased by a DC bias current and converts the varying magnetic field into an analog read signal (in volts). The read signal is proportional to the varying resistance in the MR element of the transducer 10 and is representative of the data stored within the track 25 from which the transducer 10 is reading.
Because MR elements are positive temperature coefficient devices, increases in the temperature of MR elements cause an increase in the resistance of the MR elements. Since the read signal (in volts) is proportional to variations in resistance of the MR element multiplied by the bias current and since the bias current is a constant DC current, whenever the temperature of the MR element is increased, a thermal signal is generated which adds to the value of the read signal.
One of the variables which generates thermal signals results from the presence of foreign particles or other aberrations on the surface of the disk. These foreign particles and aberrations are known as asperities. Collisions between the asperities and the transducer 10 cause the transducer to heat up. The increase in temperature resulting from the collisions between the asperities and the transducer 10 causes an increase in the resistance of the MR element. Since the bias current is constant, the resulting voltage appears to be greater than the voltage that should be present based upon the data stored on the magnetic disk. The additive signal resulting from the increase in temperature of the MR element is known as a thermal asperity.
Thermal asperities can cause unwanted increases in bit error rates. In some instances, the increases in bit error rates are so dramatic that they can cause a severe data loss. Accordingly, it would be desirable if, in addition to solving the problems described above, a system could be developed which would minimize and/or eliminate thermal asperities.
In summary, it would be desirable to develop a transducer "positioning" system: (1) having a very fast response time and which is not subject to the mechanical limitations associated with microactuators; (2) which is capable of reading from a location of a track based upon the strength of a magnetic signal stored on the disk rather than solely a position relative to the track centerline; and, (3) which can minimize and/or eliminate the affects of thermal asperities.